Try SoCal Trucks in San Diego.
http://socaltruck.net/
I purchased a used Callen from them, for my Mazda B4000, earlier this year. It's in great condition. I paid $800. People go in and sell pick up shells for resale all the time, so send them an email and let them know what you're looking for.
I had them install it and put in some additional wiring for an outlet in the back. When I had some problems they redid everything and were very good on the repairs under warranty.
There are also a fair number of these for sale on craigslist in San Diego. Now that I own one, I also notice them more often in the neighborhood. There are definitely a lot around for all different size trucks, and they are very well-made.
I got mine after doing some research since I go down into Baja. I had a fiberglass leer shell on my last truck, but by the time I got to my campsite the gear in back was always covered in dust. I don't have that problem with the Callen. It is really well sealed.
Good luck. They are bombproof camper shells.
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